Monday, November 8, 2010

California PDSI Trends

The above chart shows the Palmer Drought Severity Index (PDSI) for California in the month of September from 1900 until 2010 (with a quadratic trend line).  The data come from NOAA.  Other summer months look similar.

The trend towards more drought in the last few decades is consistent with the idea that global warming is causing more drought in California, but that this was masked before 1990 or so by a trend of increasing El-Nino's.  Again, the first principal component of global PDSI shows a marked drying of the US southwest:

Not only is California getting more drought prone on average in recent decades, it's also getting more volatile.  You can see this in the graph at the top, with a period of lower fluctuations mid century followed by big spikes up and down after about 1980.  Making a graph of the root-mean-square fluctuation from trend over the prior 15 years confirms this impression:

So if present trends continue and intensify, as expected, California faces a future of more drought, interspersed with very wet years and floods.  This matters, as California has a large and rapidly growing population, and is a major agricultural producer (producing more than half of the nuts, fruits, and vegetables in the US).

At this time, the increasing drought in California has not been sufficient to overcome the ongoing rise in crop yields due to improving agricultural technology.  For example, here are California corn yields (from NASS).  One can argue that the stagnation after 1990 might have been due to poorer weather, but if so the agricultural industry found some way to keep advancing in the last few years.

Definitely something to watch in coming decades, however.

This post is part of a series on the future of drought.


david said...

This is exactly what I was waiting for. Great analysis and thanks.

Going Green said...

How much of the CA corn is irrigated? Water is important. Check out Colorado.

Emmaliu said...

Very interesting. It's a concerning question.